1. Field of the Invention
The present invention relates to a laser processing apparatus, and more particularly to a laser marking apparatus used for marking on a substrate and the like.
2. Description of the Related Art
In a manufacturing process of a semiconductor device, marking of characters and numeric characters is directly performed on a substrate using a laser in order to manage the substrates which flow through a manufacturing line.
As to an example of a laser processing apparatus used for this kind of application, a CO2 gas laser oscillator is used, and a laser beams is oscillated by a polygon mirror and directed to a predetermined place of a glass substrate, whereby marking of characters, symbols, and the like is performed. Then, in some cases, a gas nozzle for spraying N2 gas is provided for a place to be marked with the purpose of avoiding cracks and with the purpose of cooling the place (for example, refer to JP 11-005181A(pp. 3-4, FIG. 1)).
A problem of a conventional laser processing apparatus used for marking resides in that minute particles or dusts, which are generated secondarily, contaminate a surface of a glass substrate in a process of forming a large number of concave parts in the surface of the substrate by laser ablation.
The inventor has found that: when minute particles, each of which is generated through a process of laser marking and has a diameter of approximately from 0.1 μm to 1 μm, adhere to a main surface of a glass substrate, it is difficult to remove the particles in general surface cleaning; when a thin semiconductor film with a thickness of 1 μm or less is formed on the surface, minute convex portions are formed; and as a result, characteristics of thin film transistors (TFTs) vary among elements. Further, the inventor has found that, when thermal processing such as laser annealing is performed to the semiconductor film, a part of the semiconductor film, to which the processing has been performed, bursts out and flies, as a result, a minute hole with a diameter of approximately from 0.2 μm to 2 μm is formed. This causes dispersion in the characteristics of TFTs among elements. Further, abnormal shapes such as the above-described convex portions and holes cause heat generation due to a concentration of the electric field at the portions, which leads to characteristic abnormality and decrease in reliability in the TFT.
Of course, it is considered that marking is performed to a rear surface (opposite surface to the main surface over which a film such as a semiconductor film is formed) of a glass substrate. However, a marking portion comes to have unevenness through ablation, and therefore, a problem arises, when the glass substrate is placed on a flat surface, the flat surface is not in parallel with the main surface of the substrate, which poses an obstacle in a manufacturing process of a semiconductor device. For example, when a substrate is to be fixed over a mounting base of an exposure apparatus, a glass substrate is warped, which leads to a defect that accurate patterns are not exposed.